1. Field of the Invention
The present invention relates to an electric power steering apparatus for lightening the steering force of a driver by applying the drive force of an electric motor directly to a steering system.
2. Description of the Related Art
One known electric power steering apparatus employs a differential transformer as a steering torque sensor. In this electric power steering apparatus, a clockwise or counter-clockwise steering torque generated by turning a steering wheel is detected as an electric differential output generated in the differential transformer. The magnitude of the steering torque is detected in the form of an absolute value of the differential output while the direction of the steering torque is detected in polarity (.+-.) of the differential output.
Another known electric power steering apparatus employs an operational amplifier as a steering torque detector, which amplifier is driven by a single power supply of 5 V. The operational amplifier is set to have a virtual ground point of 2.5 V so that a differential output of 0 in the differential transformer (steering torque being 0) is detected as a detection output of 2.5 V, the differential output of negative polarity (steering torque being in counter-clockwise direction) is detected as 0 to 2.5 V, and the differential output of positive polarity (steering torque being in clockwise direction) is detected as 2.5 V to 5.0 V.
In the steering torque detector, the state in which no steering torque is applied to the steering wheel (steering torque being 0) is set as a neutral point and the output of 2.5 V of the operational amplifier, corresponding to the neutral point, is set as a center value.
Still another known electric power steering apparatus is provided with a direction inhibitor for inhibiting, on the basis of a steering torque signal, the feed of a motor control signal to a motor drive to thereby determine whether or not a direction signal of the motor control signal, outputted from a control system for controlling the drive of the electric motor, and a direction signal outputted from the direction inhibitor are consistent. In the case of inconsistency, the assist by the electric motor, as opposed to the steering direction, is inhibited.
The direction inhibitor is set with two action reference values corresponding to the detected outputs of the steering torque detector in the clockwise and counter-clockwise directions. When the detected output in the clockwise direction exceeds the action reference value in the counter-clockwise direction, the electric motor assist in the counter-clockwise direction is inhibited. When the detected output in the counter-clockwise direction exceeds the action reference value in the clockwise direction, the electric motor assist in the clockwise direction is inhibited. With this construction, the electric motor assist is inhibited in response to a motor control signal outputted from a control system, when a CPU (or microprocessor) constructing the control system is troubled, for example, to invite the inconsistency between the direction signal, as detected by the direction inhibitor, and the direction signal of the motor control signal, as outputted from the control system.
As a result, an unnatural behavior of the vehicle is prevented by inhibiting such an electric motor assist against the will of the driver that the electric motor will assist in the counter-clockwise direction although the driver is turning the steering wheel in a clockwise direction.
In the conventional electric power steering apparatus, however, the assist time period by the electric motor for a torque value less than a predetermined value (e.g., 30 kgf.multidot.cm) is shortened where the steering force is set heavily in a high vehicle speed range. As a result, when achieving a steering damper effect by making an assist in the direction opposed to the torque direction, there arises a problem such that a reduced level of damper effect is produced, because the motor output is inhibited by a direction discriminator of the control system when the steering torque is at a predetermined or higher value and when the motor output direction and the torque value direction are different.